1. Field of the Invention
This invention relates to a development apparatus for developing electrostatic latent images using an electrophotographic apparatus and, more particularly, to a developer roller for a nonmagnetic one-component development system (hereinafter abbreviated as a "one-component developer").
2. Description of Related Art
Some electrophotographic photocopiers, printers, and facsimile machines use a one-component developer to make the development unit compact. In such a development apparatus using a one-component developer, a developer roller lies close to or in contact with a drum-shaped latent image carrier (photosensitive body) on which an electrostatic latent image is formed. Toner from a toner cartridge is supplied on the surface of the developer roller by a toner supply roller. The supplied toner is made thinner or smoothed by a doctor blade, for restricting the thickness of the toner layer disposed close to the surface of the developer roller, according to the revolution of the developer roller, and is triboelectrically charged to a predetermined polarity. Toner that has passed the doctor blade is transported to or in contact with the surface of the developer roller and is held by its charges, and is then conveyed to a development zone at which the developer roller and the photosensitive body meet with each other. The toner is then transferred onto the electrostatic latent image formed on the surface of the photosensitive body, thereby visualizing the electrostatic latent image.
FIG. 8 is a cross sectional view of a conventional developer roller. As shown in FIG. 8, the developer roller is typically constituted of a rubber elastic layer 5 formed at the outer round surface of a shaft 3. The surface of the developer roller i is ground so as to have a ten point mean roughness Rz of about 4 to 10 micrometers, to obtain mean toner thickness of 20 micrometers, which result in an excellent print density. The mean toner thickness necessary to obtain an excellent print density may vary depending on the kinds and amounts of carbon used as a coloring material for the toner, and the bulk density of toner particles in a toner layer. A spherical shaped toner is used, which is obtained by suspension polymerization of a binder polymer with a coloring material. For example, styrene-butyl acrylatc copolymer of 100 parts by weight is used as the binder polymer. Carbon black (a trade name, made by Denki Kagaku K.K.) of 8 parts by weight, low molecular weight polypropylene of 4 parts by weight, and dye containing metal of 2 parts by are used as the coloring material, and which are compounded and formed.
The developer roller 1, however, during a printing operation, rubs the toner supply roller, the doctor blade, and the photosensitive body with contact pressure thorugh the toner. Therefore, the roughness on the surface of the developer roller becomes worn out, and the surface roughness Rz falls 1 to 2 micrometers when about 2,000 sheets have been printed, thereby reducing the toner thickness after the doctor blade is applied, to about 10 to 15 micrometers, thereby possibly rendering the printing density lower.
The degree of such abrasion depends on the abrasion resistance of the rubber elastic material of the developer roller 1 or on the form of the toner particle. In the case that silicone rubber is used, the roller would have worse abrasion resistance than if urethane rubber were used; in the case of toner made by pulverizing a lump of toner material, the roller has much worse abrasion resistance than in the case when the spherical toner is made by polymerization.